Photo-sharing site Instagram has defended its rules on nudity after a daughter of Hollywood stars Bruce Willis and Demi Moore staged a topless protest over the app's guidelines.

Scout Willis posted pictures of herself on Twitter roaming New York's streets without a shirt or bra after her Instagram account was suspended when she shared a photo featuring two topless women.

The 22-year-old has been backed by pop superstar Rihanna, who reportedly left Instagram herself after several of her racy photos were removed from the site.

Instagram today said its guidelines put "limitations on nudity and mature content" and a lmost all of the photos removed from the site follow complaints by other users.

A spokeswoman said: "We try hard to find a good balance between allowing people to express themselves creatively and keeping Instagram a fun and safe place.

"Our guidelines put limitations on nudity and mature content. Once content is reported to us, we review it and remove if it violates our policies."

Instagram's terms of use state that users may not post "violent, nude, partially nude....pornographic or sexually suggestive photos or other content via the service."

The company's co-founder Kevin Systrom has said the rules are the same for celebrities and other users.

He told BBC Newsbeat: "Our goal is really to make sure that Instagram, whether you're a celebrity or not, is a safe place and that the content that gets posted is something that's appropriate for teens and also for adults.

"We need to make certain rules to make sure that everyone can use it."

Alongside Willis's topless protest pictures, she tweeted: "What Instagram won't let you see. FreeTheNipple" and "Legal in NYC but not on Instagram".

In a blog post on the xoJane website, Willis explained the reason for her protest, claiming Instagram deleted her account when she posted a photo of herself in a sheer top and a photo of a jacket she made that featured two topless friends.

"My situation was in no way unique," she wrote in her blog.

"Women are regularly kicked off Instagram for posting photos with any portion of the areola exposed, while photos sans nipple - degrading as they might be - remain unchallenged.

"So I walked around New York topless and documented it on Twitter, pointing out that what is legal by New York state law is not allowed on Instagram.

"What began as a challenge to Instagram and its prejudiced community guidelines became an opportunity for dialogue."

"I understand that people don't want to take me seriously," she went on.

"Or would rather just write me off as an attention-seeking, over-privileged, ignorant, white girl.

"I am white and I was born to a high profile and financially privileged family. I didn't choose my public life, but it did give me this platform. A platform that helps make body politics newsworthy."

Barbadian singer Rihanna, who tweeted her support for Willis, reportedly closed her Instagram account last month after a number of racy photos taken for French magazine Lui were removed from the site.

Vogue creative director Grace Coddington also had her Instagram account suspended temporarily after she posted a self-portrait sketch of her bathing topless.

According to reports, Instagram said the decision to block the account was an accident, and that a mistake was made while processing a complaint against the photo.

Willis has said that for every "nasty" comment she had received following her protest, there were "10 more of support, appreciation, and empowerment".

"I am not trying to argue for mandatory toplessness, or even bralessness," she added.

"What I am arguing for is a woman's right to choose how she represents her body - and to make that choice based on personal desire and not a fear of how people will react to her or how society will judge her.